The synthesis of star polymers is growing in commercial interest. Star polymers have many applications such as, inter alia, surfactants, thermoplastic elastomers, lubricants, rheology modifiers, and viscosity modifiers or control agents. In fact, some star polymers are now considered by many to be state-of-the-art viscosity modifiers and oil additives.
Synthesizing star polymers having a narrow polydispersity (i.e., uniform arm length) and a readily determinable number of arms and arm molecular weights is a long standing need. Considerable efforts have been made to prepare star-like structures having arms made of various materials and ranging in number from 2 to greater than 250. To date, three major techniques or processes have been described and/or used for synthesizing star polymers. These three techniques are typically distinguished from each other by (1) the use of multifunctional linking agents (also known as the “core and arm first” method); (2) the sequential copolymerization/linking with a divinyl monomer (also known as the “arm first”method); and (3) the use of multifunctional initiators (also known as the “core first” method).
With respect to the “core and arm first method,” multifunctional linking agents have proven useful in conjunction with anionic-polymerization techniques for preparing homo-, block-, and hetero-arm star polymers. Multifunctional linking agents have also been used in conjunction with carbocationic polymerization techniques to prepare four-arm poly(isobutyl vinyl ether) stars, and multi-arm polyisobutylene stars. In the latter instance, the polyisobutylene stars can be prepared by the hydrosilation of allyl-terminated polyisobutylenes with siloxane cores.
Linking living-polymer chains with divinyl monomer(s) is also well known and has been used for the synthesis of multi-arm stars by anionic, cationic, and group-transfer polymerization techniques.
Using multifunctional initiators for the synthesis of multi-arm star polymers has not, however, been as thoroughly developed in certain respects. For anionic polymerization systems, the use of multifunctional initiators is somewhat limited due to relatively poor initiator solubility in hydrocarbon solvents. However, at least two studies have shown that hydrocarbon-swollen polydivinylbenzene can be used as a multifunctional anionic-polymerization initiator. Unfortunately, the polydispersity of the star polymers was rather broad in each of these studies. Other studies have used a hydrocarbon-soluble trifunctional initiator for preparing homo-, block-, and functionalized star polymers.
The solubility problems of multifunctional initiators are generally less severe with carbocationic polymerization systems. And carbocationic polymerization techniques are readily employable in synthesizing multi-arm stars.
Among the many advantages of using multifunctional initiators with carbocationic polymerizable monomers such as isobutylene, is that the resultant stars are “directly telechelic.” By directly telechelic, it is meant that the resultant star will have a functional group at the end of each arm upon terminating the polymerization reaction. That is, chain-end functionality of the arms is preserved during formation of the star. A chain-end functionality is also known as the functionality at a terminus of an arm. In comparison, other techniques for synthesizing star polymers require at least an additional processing step to provide a chain-end functionality.
There has been a growing interest in star polymers that have multiple polyisobutylene (PIB) arms. For example, Kennedy et al. U.S. Pat. No. 5,395,885 describes the synthesis of star polymers having multiple PIB arms and polydivinylbenzene (PDVB) cores using cationic “arm-first” synthesis techniques. Because the structure of PIB is readily characterized and contains no sites of unsaturation, these PIB-based stars are believed to be useful for a variety of applications such as motor-oil additives and viscosity-index improvers. However, their potential is still being evaluated and tested, and in motor-oil additives where certain properties such as, for example, sensitivity to oxidation, is of critical importance, the possibility remains that, because of using crosslinked aromatic cores such as PDVB, the PIB-PDVB stars may not be highly desirable for such use.